In certain applications it is desirable to provide a sealed connection between two members. In one such application, it may be desirable to seal a tube within a housing, for instance, sealing an inlet tube within the housing of a differential pressure flow meter/sensor. In traditional methods, such a seal may be achieved by brazing or bonding a sealing member between the tube and the housing. Such methods may be difficult, particularly if the tube and housing are made of different materials, and may also be inconvenient or expensive. Alternative mechanical connections may also be difficult to implement, given the size tolerances of the required fitting. In addition, operating conditions, mechanical fatigue and thermal expansion differences may cause such connections to fail.
An improved sealing connection may be achieved by using a shape memory alloy (SMA) sealing member. Shape memory alloys (SMAs) are materials that have the ability to return to a predetermined shape when heated above a transformation temperature (i.e. “hot” state). When below their transformation temperature SMAs exist in a martensitic phase which has a low yield strength that can be easily deformed into a new shape (i.e. “cold” state). Once heated above the transformation temperature, the SMA reverts back to an austenitic crystal structure and recovers its previous shape. If the SMA faces any resistance during transformation it can generate large forces, which can be used to provide a sealing interface.
This behaviour allows one to deform a sealing member to a shape that may be passed easily around a first member for sealing between the first member and a second member, and then activate a subsequent sealing action by heating it, so it attempts to return to its original shape between the two members. This may make SMA sealing members simpler to install and more cost-effective than the aforementioned methods.
Additionally, when above the transformation temperature, the SMA may exhibit pseudoelastic behaviour. This advantageously allows the SMA to absorb high strains whilst maintaining a certain stress or sealing force, which gives such sealing members a high fatigue resistance and allows their use in demanding environments. Such effects are well known in the art and are described in detail in, for example, “Phase Change Behaviour of Nitinol Shape Memory Alloys” by J M Gallardo et al.—Advanced Engineering Materials 2002, 4, No. 7 and US Patent Application 2002/0074742 (Quoiani).